In the days since the Feb. 14 massacre in Parkland, Fla., it’s become clear that many Americans — foremost among them the students who made it out of Marjory Stoneman Douglas High School alive — have lost their patience with feckless lawmakers and AR-15 fetishists. The empty gestures that dominated the gun-rights debate for decades have been supplanted, at least for now, by the galvanizing fury of “we call B.S.!”

If this is a moment that demands unambiguous language, Roxanne Dunbar-Ortiz is well-positioned to meet it. A San Francisco resident and the author of “An Indigenous Peoples’ History of the United States,” Dunbar-Ortiz is a knowledgeable, unflinching writer whose new book takes a close look at the origins of this country’s aberrant relationship with guns.

“Loaded: A Disarming History of the Second Amendment” isn’t uniformly convincing; occasionally, Dunbar-Ortiz presents some eccentric, unverifiable theories. But her main arguments — that the Second Amendment enabled slave owners and many settlers to persecute black and Native people, and that these devastating power dynamics continue to shape our society — are unassailable.

The Second Amendment, adopted in 1791, allows that “a well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed.” Generations of activists, gun owners and jurists have disagreed about how James Madison’s words should be interpreted in a modernizing world. But to Dunbar-Ortiz, these discussions are often lacking in context.

“The elephant in the room in these debates has long been what the armed militias of the Second Amendment were to be used for,” she writes. “The kind of militias and gun rights of the Second Amendment had long existed in the colonies and were expected to continue fulfilling two primary roles in the United States: destroying Native communities in the armed march to possess the continent, and brutally subjugating the enslaved African population.”

Dunbar-Ortiz demonstrates that the violence sanctioned by the Second Amendment was a key factor in transforming America into a “militaristic-capitalist” powerhouse. As the fledgling nation embarked on its relentless westward expansion in the 19th century, the labor of millions of slaves was making America exponentially richer and more powerful.

Meanwhile, America’s nascent gun culture was growing in tandem with the country’s increasingly sophisticated publishing and firearms-manufacturing industries. Pulp novels romanticized ex-Confederate soldiers and rifle-wielding frontiersmen. Brand-name weapons acquired special status. “In the two decades after the Civil War,” Dunbar-Ortiz writes, “the Winchester rifle was fetishized for killing Indians, and the Colt revolver for outlawry. In the process, gun violence and civilian massacres were not just normalized, but commercially glorified, packaged, promoted, and mass marketed.”

Dunbar-Ortiz also chronicles the disturbing evolution of National Rifle Association. For years, the NRA supported common-sense regulations, but in the 1970s, new leadership moved the group toward Second Amendment absolutism. Today, as a matter of course, the NRA uses fear and lies as fundraising tools. At a conservative conference held the week after the Parkland shootings, Wayne LaPierre, the NRA’s leader, warned that a “tidal wave of new European socialists” — California Sen. Kamala Harris among them — is conspiring to take away America’s guns.

These are infuriating tactics, but Dunbar-Ortiz suggests that gun control advocates would be well-served if they spent more time on constitutional questions. “Perhaps reverence for the Second Amendment and the privileging of individual rights over collective rights, not the N.R.A., are the source of the problem with enacting firearms regulations,” she says. If its dark origins were more widely accepted, Americans might be less supportive of unfettered Second Amendment rights. This could reorder the political landscape.

Alongside Dunbar-Ortiz’s compelling arguments are a few questionable theories. She implies that the martial ethos of the Pentagon and the CIA is somehow linked to school shootings and other stateside massacres: “There is no way to prove a correlation between war crimes — the U.S. bombing of civilian populations and their infrastructure — and domestic mass shootings, but the relationship does not appear to be random.” There might be an interesting argument here, but it’s poorly developed; if even the author concedes that such an assertion can’t be fact-checked, it’s not worth including in a book that intends to be a serious work of history.

The same can be said of her supposition about the 2007 Virginia Tech shooting, in which Seung-Hui Cho murdered 32 people before killing himself. “One could speculate that the shooter, Cho, may have taken note” that a 400th anniversary commemoration of the founding of Jamestown, Va., also held in 2007, made “no mention, much less apologies, for the murder of Powhatan farmers and the destruction of their homes and fields.” This is bizarre conjecture.

Dunbar-Ortiz shares a few autobiographical details that will be familiar to readers of her memoir “Outlaw Woman.” In the early 1970s, she was part of a New Orleans activist group that was surveilled by police and federal authorities. In response, the group bought several guns and “spent hours every day breaking down, cleaning, oiling, and polishing our weapons. We took turns loading shotgun shells. We had fallen under the spell of guns.”

Dunbar-Ortiz’s unhealthy relationship with guns ended after about two years. America’s has lasted a lot longer, but in the wake of Stoneman Douglas, there might be reason, at last, for some very cautious optimism.

Kevin Canfield has written for Bookforum, Film Comment and other publications. Email: books@sfchronicle.com